Influence has been a hot topic here for the past few months. But what determines someone’s influence on Twitter? Number of tweets? Number of followers? Number of followers’ followers? Researchers from Northwestern University seem to have found the answer, and it’s a lot more complicated than who follows who.

Ramanathan Narayanan, a Ph.D candidate at Northwestern University, and his professor Alok Choudhary developed a project that tracks influence on Twitter based on specific topics. For example, LeBron James might wield a lot of influence about the topic “basketball” but very little about “politics.” The research project started in May, and the website, pulseofthetweeters.com, launched at the beginning of this month.

So what defines influence? “That’s a difficult question to answer,” Choudhary says. His algorithms calculate a tweeter’s influence based on the actions (retweets, direct messages and “@” responses) his tweets inspire in his network, defined as his followers and his followers’ followers. Time is also a factor. However, unlike Fast Company‘s project, the algorithm does not account for whether a user clicks on links in a tweet.

Choudhary noted that number of followers and number of tweets don’t define a person’s Twitter influence. Just as there is spam e-mail, there are many junk tweets on Twitter and the team’s algorithms eliminate these conversations from the sphere of influence.

The team specializes in data mining technology — but with Twitter, some changes had to be made to conventional methods. “One has to take a step beyond traditional data mining because it’s not, so to speak, proper English,” he says. “The assumption is that it’s grammatically correct for traditional data mining.”

The topics and influential tweeters update in real time on the site, and the program also breaks down the sentiment of the tweet. Choudhary says the program determines whether a tweet is positive or negative based on past tweets, and the team continuously updates the model from what they learn. “This is not an exact science,” Chouhary admits.

Everyone who tweets on a topic is included in the ranking. Topics are derived from What the Trend?, a program that determines popular trends on Twitter, and Choudhary says that they plan on adding a feature where users can select any topic of interest and see who is influencing the Twitter conversation.

The results can be surprising. For the topic “obama,” the most influential tweeter is nprpolitics, which also happens to be the tweeter with the most followers in this case. (Obama has 16% positive tweets, 24.2% negative and 49.8% neutral.) For the new HBO series, Boardwalk Empire, HBO’s own Twitter feed ranks a mere sixth in terms of influence on the topic, “boardwalk empire.” However, when the topic includes the hashtag “#boardwalkempire”, the network is the top influencer. And we’re still wondering why the hip hop artist Common is the top influencer on retired football star “jerry rice.”